1. Field of the Invention
This invention relates to a rechargeable lithium cell which is adapted for use as a mobile power source and a power source for memory backup. The invention also relates to a process for making an anode particularly suitable for use in the rechargeable lithium cell.
2. Description of the Prior Art
A recent progress of technology in the fields of electronics expedites miniaturization of electronic devices. As a power source for such devices, there has been an increasing demand of cells which are small in size and light in weight with a high energy density. To this end, attention has been directed to rechargeable lithium cells using a lithium anode. Such cells have been studied worldwide.
However, when a rechargeable lithium cell using metallic lithium as an anode is subjected to repeated charge and discharge cycles, there arise disadvantages that the lithium is partly divided into fine pieces during the cycles and that dendrite of lithium is grown on the surface of the anode, thereby causing the anode to be deformed with the cycle life of the cell being very short.
In order to overcome the disadvantages, use of alloys and metal oxides as the anode has been investigated, including lithium and aluminium alloys, lead alloys occluding lithium therein, niobium pentoxide doped with lithium, titanium dioxide having an anatase crystal structure. Especially, for preventing the formation of lithium dendrite crystals, the metal oxides are superior to the alloys. Titanium dioxide exhibits a fiat charge and discharge curve at a potential of approximately 1.5 V, with the great possibility that titanium oxide can be used as the anode of a rechargeable lithium cell. Alternatively, Japanese Patent Publication No. 63-1708 proposes the combination of an anode made of titanium oxide and a cathode made of manganese dioxide. However, the cell using such a combination as set forth above has not been put into practice yet because the potential is so low as 1 V and the cycle life is short.
As is known in the art, titanium dioxide has several types of crystal structures including blue kite, anatase and ruffle structures. Titanium dioxide having the anatase and rutile structures has been studied as the anode of the lithium cell as set out in "Electrochemistry 46. NO (1978) p. 407). The anatase structure exhibits a higher electrode capacity than the rutile structure and, thus, anatase-type titanium oxide has been rated high. Eventually, when used as the anode, anatase-type titanium oxide exhibits a higher capacity than rutile-type titanium dioxide. However, these titanium dioxides are liable to suffer breakage of crystal lattices when lithium ions are doped and un-doped, so that the cycle life characteristic of the resultant cell is not always good. Thus, the cell is degraded in capacity on repetition of the charge and discharge cycles.